The present invention relates to an improved shape memory metal (SMM) actuator which can readily be adjusted to compensate for cyclic creep.
SMM actuators typically utilize an actuator element made of a shape memory metal such as a nickeltitanium alloy. Such materials are characterized by a transition temperature at which they transform from a martensitic state to an austenitic state. During this transformation, the SMM actuator element can do work. It is well known that as SMM actuator elements are cycled in temperature the effective length of the SMM element gradually increases, both when in the austenitic state and when in the martensitic state. Such a gradual increase in length is typically referred to as cyclic creep. If not corrected, cyclic creep can adversely degrade the effective stroke of the SMM actuator. In addition, it is widely recognized that SMM actuator elements should be protected from excessive stresses if the life of the actuator element is not to be unduly shortened. U.S. Pat. No. 4,490,975 addresses problems related to both cyclic creep and overload protection for SMM actuator elements.